1. Field of the Invention
The present invention relates in general to subcutaneous electrode structures and, more particularly, to an improved electrode structure useful as a chronic implant for taking electrocardiograms of active animals.
2. Description of the Prior Art
Radiotelemetry has long been applied to physiological monitoring. Biotelemetry systems are especially suited for the monitoring of unrestrained animals. Implanted biotelemetry systems have been employed to monitor temperature, pressure and biopotentials. Heretofore, there has not been a suitable internal electrode for providing electrocardiograms (ECG or EKG) in active animals wherein the electrode is chronically implanted.
Conventional ECG electrodes tend to produce movement artifacts (movement generated noise) so that it is difficult to obtain quality data when the test subject is active. Therefore, it is desired to obtain an improved chronically implanted, subcutaneous electrode structure which has reduced electrical noise associated therewith.
In the past, designers of external electrodes were concerned with maintaining a high amplifier input impedance to electrode impedance ratio to prevent loading, signal distortion and extraneous 60-cycle noise. Therefore, they chose external electrodes with large distributed surface areas in order to compensate for the high contact impedance caused by the cornified epithelium of the skin. Of course, as the electrode area was increased, the contact impedance was reduced. The problem caused by the skin barrier was greatly reduced with the advent of electrolytic jellies and pastes as well as electrode designs featuring pointed projections to pierce the cornified epithelium. The contact areas of modern external electrodes, as a result, are often much smaller in appearance than the old style clinical plate electrodes. 60-cycle stray noise is seldom a problem with internal electrodes. Further, internal electrodes do not encounter a cornified epithelium. Investigators such as Geddes and Baker have stated (Med & Biol. Engng., Vol. 4, pp. 439-450, Pergamon Press, 1966) that subcutaneously implanted stainless steel needle electrodes have a low enough impedance to prevent amplifier loading and signal distortion.
Prior suggestions on how to reduce the problem of movement artifacts seem to stress that a major requirement is to protect, to isolate, to stabilize, and/or to limit the electrode-electrolyte interface from disturbances. The interface involves a half cell potential that is imparted to an electrode through reversible oxidation--reduction reactions. This gives rise to an alignment in charge distribution. The process is discussed in most chemistry textbooks and is commonly known in the context of batteries. When disturbances of this electrochemical interface occur, the half cell potential of one electrode relative to the other is altered and these transient differences in potential between the two electrodes are believed to be the cause of artifacts.
Many prior art ECG electrode designs have been predicated on half cell considerations. The majority of the available information is presented in the context of external ECG electrodes. In the past, the two most often emphasized approaches for the design of external electrodes are to use low mass to reduce problems of inertia and to use recessed electrodes in combination with an electrolytic substance that allows for lifting the interface away from and protecting it from the movements of the underlying skin. For example, it is stated in "Principles of Applied Biomedical Instrumentation," 2nd Edition, L. A. Geddes and L. E. Baker, John Wiley & Sons, New York, 1975, pp. 226-227, that when biopotential measurements are to be made on active subjects "it is important to make the electrodes as small and light as possible."
By far the most prevalent means for attempting to solve the problem of movement artifacts in internal electrodes was to provide an electrode of a very small surface area which could be anchored quite securely against the tissue. Electrodes of this type include small hooked wires, needles, or wire loops which are discussed in "Medical Instrumentation," John G. Webster, Houghton Mifflin Company, Boston, 1978, pp. 250-255.
It is also known from the prior art to employ, as a subcutaneous electrode, a porous plate composed of a tissue-compatible implantable material such as platinum. Such an implantable electric terminal for organic tissue is disclosed in U.S. Pat. No. 4,011,861, Mar. 15, 1977. The porous electrode permits body electrolytes and/or tissue containing blood capillaries to contact the electrically conductive material of the electrode through the porous material or layer. Electrical connection is made to the electrode by means of a rod bonded to the disc in the central region thereof and extending normally from a major face of the electrode. Such an electrode structure is unsuited for a chronically implanted, subcutaneous electrode for several reasons. Its physical configuration and size when placed between layers of tissue would place pressure on the tissue that would result in time to necrosis of the tissue and eventual rejection or displacement in location. The connection of the rod structure to the porous material and the connection of the rod to some necessary lead conductor are not stress relieved and flexible. Thus with movement, the interface between the tissue and electrode would be distorted and result in artifacts and tissue damage. There are no indicated methods or apparatus for suturing the electrode in place so as to insure a fixed initial location before tissue imbedding occurs. Porous materials are hard to clean and to keep clean during a surgical implant. Should infection occur, these pores would serve as a reservoir for the infection; a massive tissue would ensue and wall off and isolate the electrode structure into a high impedance dry sack-like formation.
It is also known from the prior art to employ implantable electrodes for stimulating a selected portion of the spinal cord of an animal. Such implantable electrodes included a plurality of flat plate-like electrode portions with upturned lips. Electrodes of this type are disclosed in U.S. Pat. No. 3,724,467 issued Apr. 3, 1973. Also plate-shaped electrodes are mentioned in U.S. Pat. No. 3,089,483 issued May 14, 1963.